1. Field of Invention
The disclosure relates generally to methods and apparatus for printing viscous material, such as solder paste, onto a substrate, such as a printed circuit board, and more particularly to a method and apparatus for improving the accuracy of print pressure or force applied by squeegee blades of a print head on a stencil, and for applying an accurate print pressure during production.
2. Discussion of Related Art
In a typical surface-mount circuit board manufacturing operation, a stencil printer is used to print solder paste onto a printed circuit board. A circuit board, broadly referred to as an electronic substrate, having a pattern of pads or some other conductive surface onto which solder paste will be deposited, is automatically fed into the stencil printer. Small holes or marks on the circuit board, called fiducials, are used to align the circuit board with the stencil or screen of the stencil printer prior to the printing of solder paste onto the circuit board. The fiducials serve as reference points when aligning a circuit board with the stencil. Once a circuit board has been aligned with the stencil in the printer, the circuit board is raised to the stencil by a substrate support, e.g., a table having pins or other work holders, and fixed with respect to the stencil. Solder paste is then dispensed by moving a wiper blade or squeegee across the stencil to force the solder paste through apertures formed in the stencil and onto the circuit board. As the squeegee is moved across the stencil, the solder paste tends to roll in front of the blade, which desirably causes mixing and shearing of the solder paste to attain a desired viscosity to facilitate filling of the apertures in the screen or stencil. The solder paste is typically dispensed onto the stencil from a standard solder paste supply cartridge. After the print operation, the board is then released, lowered away from the stencil, and transported to another station within the printed circuit board fabrication line.
During a print cycle, as described above, the squeegee is moved across the stencil to force solder paste or any other viscous material through apertures formed in the stencil. FIG. 1 schematically illustrates a print head, generally indicated at 200, having a squeegee blade 202. In one embodiment, the squeegee blade 202 may be secured to a squeegee blade holder 204 in a position in which the squeegee blade may be disposed vertically or at an angle with respect to a stencil 206 to force solder paste through the apertures (not shown) of the stencil. In one embodiment, the print head 200 may include a first movable plate 208 and a second movable plate 210 that is connected to the first movable plate. The first movable plate 208 may be secured to a frame (not shown) of the print head 200 by two linear bearings, each indicated at 212. The first movable plate 208 may be configured to move up and down by means of a lead screw 214, which is driven by a motor (not shown) provided in the print head 200. The arrangement is such that the lead screw 214 threadably engages a lead nut 216 secured to the first movable plate 208 to move the first and second movable plates 208, 210 along a path defined by the linear bearings 212. As shown, the second movable plate 210 may be connected to the blade holder 204. In a certain embodiment, a compression spring 218 may be disposed around the lead screw 214 to provide a resistance force between the first movable plate 208 and the second movable plate 210.
Accurate printing is dependent upon the print head 200 being able to apply a constant pressure on the stencil 206 during a print operation, which is often difficult to control. One cause of inconsistent pressure application is due to inadequate support of the circuit board. Specifically, the support tooling (e.g., pins or flexible tooling) may not adequately provide support to the circuit board during the performance of a print operation. Other causes of inconsistent pressure may be associated with variations associated with the spring constant of the compression spring 218 and unwanted friction created by the linear bearings 212, which either alone or together may make it difficult to determine whether a desired pressure or force is being accurately applied. Yet another cause is that the spring constant of the squeegee blade 202 may effect the force upon which the squeegee blade engages the stencil 206.
With reference to FIG. 1, a sensor 220 may be provided to a home position and/or a predetermined distance of the first movable plate 208 with respect to the frame of the print head 200. With the known calibration methods, e.g., replacing the squeegee blade 202 with a calibration gauge (not shown), the force of the squeegee blade against the stencil is determined by moving the first movable plate 208 a known distance, which is dependent upon the spring constant of the compression spring 218 and the friction of the linear bearings 212. Thus, if a completely rigid squeegee blade is utilized, the force of the squeegee blade 202 against the stencil 206 may be somewhat accurate, provided, however, that friction caused by the linear bearings 212 is minimal or predictable. It may be difficult to determine the force of the squeegee blade 202 against the stencil 206 when the squeegee blade is flexible, when the linear bearings 212 are not properly installed, or when the spring constant of the compression spring 218 is not to tolerance.